I took my A-levels close to 6 years ago, and my results were
distinctively average(aka, no A). I applied for Arts faculty but
didn't get it, but instead was offered a place at NTU, which I
declined because I had absolutely no interest in that course.
Let's face it, with competition not only from the A-Level people,
but also the best from Poly people, good grades along with a good
CCA record will help.
Since you are not confident, I would suggest that while u are
waiting for ur A-level results, do go to private schools like SIM
and PSB(I sincerely don't recommend my school MDIS) and take a look
at the courses offered. So that u will know what u want to study,
and in case you can't make it to the top 3 schools(SMU,NTU, NUS)
you can immediately make preparations for the course at the private
institutions.